Because of these mission, by the end of the decade, we may have mineralogical maps of Mars that are better than those of the Earth. We should also have a much improved understanding of Martian weather, particularly on scales that would matter to human visitors.

"Mars is not the Moon. There are planetary dynamics there that blow away anything we have on the Moon," says Dr Garvin. "The planet is changing - exchanging gasses with itself - the weather systems are dynamic. The temperature variations are extreme."

By 2009, Nasa hopes to have the capability for pinpoint landings and the ability to roam many tens of kilometres across the surface.

Mars sample return

But James Garvin's biggest challenge is to bring a piece of the planet back to Earth.

Signs of recent water flowing on Mars

"We have meteorites from Mars but my biggest task is to bring back a fresh piece of Mars. We know we need samples." But when?

"As soon as it is technically feasible. We have to do our homework and develop the technology we need. It will take us a decade. But when we can we will do it," he said.

But what about sending humans to Mars?

Sean O'Keefe, Nasa's new Administrator, has a vision that human explorers will move out into deep space.

"That's a vision not a programme, but I ask, 'what is the difference between what we are doing on Mars now and the preparation we would have to do for a manned Mars mission'. The answer is nothing.

"Our unmanned exploration of Mars is just the homework we need to be doing for any eventual manned mission. This programme is the right antecedent for man on Mars."

Dr Garvin points out that other areas of Nasa will be looking into the technologies needed to propel humans across interplanetary space, keep them alive in space and bring them back home.

And he is keen to put talk of a programme to put men on Mars into perspective.

"We will go but let's be sure what we are doing. It is a distance 1,500 times further than the distance to the Moon. In the history of human exploration there has never been such a leap as we will take from the Moon to Mars.

"To do it, we need to know more, develop better technology, and understand Mars better. That is what we are doing."